Commercial digital printing, such as high speed inkjet web printing, is rapidly replacing traditional impact printing or “plate” printing methods such as offset printing. Indeed, inkjet printing is nowadays becoming a popular way of recording images on various media surfaces, particularly paper, for a number of reasons, including, better colored image quality, capability of high-speed recording, and cost competitive. In this printing method, print media play a key role in the overall image quality and permanence of the printed images. Thus, it has often created challenges to find media which can be effectively used with such printing techniques.
In view of obtaining a superior image quality, a coated media is typically used. Such media has single or multiple image-receiving layers which are disposed onto a media substrate. Typically, the image-receiving layer is made of a coating composition which includes inorganic or organic pigments as the filler and polymeric materials as the binder, along with other functional materials. The image-receiving layer usually promotes performance of the ink receiving properties of the media such as the image quality, ink dry time and capacity for duplex. It also improves the sheets quality of the printing media. For example, the coated media show superior physical properties over uncoated media in terms of paper physical appearance such as gloss and surface smoothness. When the low cost media substrates such as the cellulose substrates, containing wood pulps are used, the image receiving layer can enhance the sheet whiteness and brightness significantly, and can also function as the barrel layer for anti-yellowing.
Coating compositions, adapted for printing papers, often include a white pigment and a binder. The function of the binder is, mainly, by forming a continuous film, to bond the pigment particles together, as well as to bind the pigments particles to the base substrate.
More often, binders, in paper coating, are negatively charged latexes (forming anionic polymers latexes) and are thus compatible with negatively charged pigments slurry without causing any precipitation of the slurry when mixed together. Examples of such binders are synthetic polymer emulsions such as styrene butadiene latexes (SBR), carboxylated SBR, styrene acrylate latexes (acrylate latex) and polyvinyl acetate latexes (PVAc).
When paper is used for inkjet printing, it is highly desirable that image receiving papers not only meet the common requirements for printing media, but also functionalize to improve inkjet characters with fast ink drying, high optical density, minimal spread (feathering or bleed) and sharp or clean edges (wicking or line edge raggedness). Though the above list of characteristics provides a worthy goal to achieve, there are difficulties associated with satisfying all of the above characteristics. Accordingly, investigations continue into developing coating formulations that provides high quality printing media.
Surface characteristics of the printing paper have a primary influence on qualities of ink jet printing. To this end, ink fixative is often included into coating compositions and applied onto paper surface. However, ink fixative is often an electrolyte from which the cationic ions adversely impact the stability of negatively charged binders (anionic polymers). Indeed, ink fixative can precipitate binders and then resulting in coating composition that cannot be coated.